Joseph, Joseph M Gwinn skrev: > Magnus, > > [email protected] wrote on 01/07/2009 01:27:52 AM: > >> Joseph M Gwinn skrev: >>> First the background: >>> >>> In some timing distribution applications, the primary source of >>> interference comes from different ground voltages in different parts > of >>> the facility, such as a ship or a megawatt radar. > > I left a useful detail out: The reference signal is a 10 MHz sinewave.
10 MHz into a transmitter. Should not be too hard to master. For some reason I feel confident in that environment. :) >> For most purposes an isolation transformer would solve this issue. The >> unfortunate signal characteristics of a PPS pulse makes this a little >> more cumbersome, but not unachievable, but it is no longer a simple >> passive device. For higher frequencies will RF chokes be an aid of >> course, but the RF choke needs "bolting down" in order to be effective, >> so that there is a common mode current for the RF choke to object to. >> However, the RF choke is not as effective with lower frequencies and >> essentially useless for DC. > > The receivers have built-in RF transformers. There is no 1PPS signal per > se, although the transformer would probably pass such a signal well > enough. What is being carried is 10 MHz. > > The problem is to devise a test and spec that ensures that the actual > implemented circuit in the receivers suffice. There are many ways to > botch this circuit. I see. It is fairly easy to induce common mode currents and DC voltages. An isolational transformer from a source and then on the other side simply DC offset or apply signal through a transformer if not directly from an amplifier. >> You should look into the telecom set of standards. If you think of it, >> they have been addressing this particular problem for ages. The words >> which probably get you right on the target is "bonding network" since >> you bond to the ground. > > This is just the sort of lead I was hoping to find. Great. >> In short, there are two grounding strategies: all gear is floating >> relative the safety ground or all gear is internally tied to the safety >> ground. There is benefits and problems with both strategies. Regardless, > >> a hierarchial star ground strategy emerges. > > In our systems, everything is tied to ground for both safety and RF > reasons unrelated to timing signals. And we do have a star of sorts, but > the story always ends up more complex than that, so it always ends up > being a somewhat random grounding grid. As always. > My problem is not safety, it is tolerance of conducted EMI. The reason I mention safety is that some people suggest solutions which does not fullfill the safety criteria in spirit or standard. It gets you into the right category of solutions. >> One document to start with is the "Qwest Technical Publication >> Grounding - Central Office and Remote Equipment Environment" at >> http://www.qwest.com/techpub/77355/77355.pdf >> >> Not to say that it is the standard of any sort, but I think it is a good > >> document to start from as it is a public source of telecom bonding >> practices to be used in many facilities, implementing existing >> international standards and involving transmitting towers (which is >> within your field). >> >> IEC 60950 should be a standard reference regardless. >> >> You should also consult Bellcore GR-1089. There are additional Bellcore >> specs, but starting with GR-63 and GR-1089 is not totally off the mark >> at least. Bellcore specs costs money, but if you need to comply there is > >> no alternative. >> >> ITU-T has a set of documents, such as the K-series of standards. You can > >> download these for free at: >> http://www.itu.int/rec/T-REC-K/e >> >> The European telecom world uses ETSI EN 300 253 as basis. They require a > >> login which you can get for free and then pull down all the documents >> you like. There is also alot of specific EMC documents for various >> contexts etc and they are all there. ETSI EMC is the TB handling them. >> >> On the military side, MIL-HDBK-419 may be a guide: >> http://tscm.com/MIL-HDBK-419A.PDF >> >> Old standard MIL-STD-188-124B: >> http://www.tscm.com/MIL-STD-188-124B.PDF >> >> Newer standard MIL-STD-1310 for ships: >> http://www.earth2.net/parts/basics/milstd1310g.pdf > > I will be doing some homework. Some of these are tomes. You could also look up ETSI EN 300 132-* and EN 300 386 which is relevant for telecom boxes. Further on is EN 300 199-* probably good to have around, but maybe not so applicable to this particular problem. What you want to transfer is similar to an E1 or E2 on an intra-office link. >> EMF due to bad conditioning for instance. >> >> There are many anecdotes and horror stories to be told on the subject. >> There are also sucesses stories to be told. > > We do have a bonding story, one that sort-of follows MIL-STD-1310, even > though the system is land based. Sounds good. Will think about levels. >> What makes the field a bit complex is that you need to think about >> failures, EMC, bonding, interference, lightning strikes (on wire, in >> tower, on building) which can cause a disparity of various indirect >> effects. It's a bit like being a time-nut. We could probably have a >> separate email list setup for that kind of discussions alone. > > Fortunately for me, I do not have to worry about lightning. That's > handled elsewhere, as all these cables are within a steel-frame building > with a lightning protection system built in. Actually, the most outer cabling links needs to be shielded or else they would intduce into cables. Cheers, Magnus _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
